40 amp fuse question; blade vs bolt down

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zacksc   100 W

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40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 06 2021 12:33pm

I am working on a project where an inverter draws about 25 amps* continuously from a 13 volt battery-based source. I am using 10 AWG wire with 30 amp blade fuses in in-line fuse holders on the 13 volt wires. (Something I got on Amazon that happened to have the connectors I wanted on both ends and seemed like it might work okay? (pictured in the reply below.)) Anyway, after about 40 minutes one of the fuses was blown. The fuse holder felt warm to touch and so I wondered if I should just replace that fuse with a 40 amp, or if I need to make more fundamental changes such as using a different fuse type and lower gauge (bigger) wire?

If I just replace the 30 amp with a 40 amp blade fuse in the same in-line fuse holder, I was concerned that maybe the fuse holder would tend to heat up and that might be bad?? I wondered if it might be more prudent to go with 40 amp bolt-down type fuse such as Baomain ANL 40 amp in a Baomain holder, or a Little fuse 40 amp Mega or...? I could mount that fuse in a small switch box that I use to precharge the inverter. I could also change to 8 AWG wire. Is that necessary? The total length of wire is just about 5 feet.

I know next to nothing about fuses and I have very little experience. So any advice about what is safest, most prudent, best, etc., is most welcome. Thanks very much!

* The project involves a Jackery E1500 Li-ion battery system which is being charging at 260 Watts. The power for that comes from a Victron Phoenix 375 VA inverter which is powered from the 13 volt source with about 25 amps on the input (13 volt) side of the inverter). The estimate of 25 amps at 13 volts is based on assuming 82% efficiency for the entire chain. Assuming 77% efficiency instead, the inferred current on the 13 volt side would be 26 amps. (13a x 26v = 338 Watts) I have note measured the current directly.
Last edited by zacksc on Dec 06 2021 4:56pm, edited 4 times in total.

zacksc   100 W

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Re: 40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 06 2021 12:34pm

Image This shows a picture of the fuse holder that got somewhat warm and which held the 30 amp blade fuse which blew after about 30 minutes of operation at 26 amps or so.
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Re: 40 amp fuse question; blade vs bolt down

Post by goatman » Dec 06 2021 10:46pm

i melt those holders at 40 amps

i recently bought one of these 40amp blue seas breakers for my boat, it trips at 34amps. it uses screw/ring connectors

https://www.amazon.ca/Blue-Sea-Systems- ... r=8-8&th=1

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Re: 40 amp fuse question; blade vs bolt down

Post by jonyjoe303 » Dec 06 2021 11:45pm

some of the atc fuses have a tendency to melt from the heat, this usually occur as low as 15 amps, I now use bussman atc fuses which can handle the heat better without melting. Also the fuse holders probably max out at 30 amps and they also start melting when use constantly at 30 amps.
For situations where I need high amps constantly I have been using ANL fuses, they are larger and can handle the amps better without melting. These require a ANL fuse holder (about 10 dollars).

I also tried using "shortstop" circuit breakers but you have to use namebrand breakers, I bought generic 30amp breakers and they were tripping at 22 amps. The 30 amp namebrand breakers work great and can handle the higher amps without heating up and I use them between my solar controller and battery which sees a constant of 27 amps. At 27 amps the 30amp atc fuse holders/fuses got so hot that you couldnt touch them, the 30 amp shortstop breakers get warm but you can keep your fingers on them.

ANL fuses
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shortstop circuit breakers
a shortstop breaker.jpg
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john61ct   100 GW

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Re: 40 amp fuse question; blade vs bolt down

Post by john61ct » Dec 07 2021 2:38am

Class T

MRBF terminal fuses

ANL fuses

Cooper Bussman, Blue Sea

not the category to shop for on price, avoid Amazon/eBay/Wally/Ali

cheap Chinese run away

Use known-good Western industrial strength vendors

zacksc   100 W

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Re: 40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 08 2021 11:54am

john61ct wrote:
Dec 07 2021 2:38am
Class T

MRBF terminal fuses

ANL fuses

Cooper Bussman, Blue Sea

not the category to shop for on price, avoid Amazon/eBay/Wally/Ali

cheap Chinese run away

Use known-good Western industrial strength vendors
Thanks all for the excellent advice. I just ordered a Blue Sea 5005 ANL fuse block and a Blue Sea 5165 ANL fuse from Peter Kennedy Yacht Services (PKYS) in Maryland. I also got a New Zealand made switch from them: a BEP 701S, so that I can switch from from a 2 Ohm precharge resistor to a direct (zero Ohm) circuit connection with a single switch, no interruption and way better and safer. Thanks for the education! No more cheap junk for me. I spoke with Peter on the phone and he gave me great advice and helped sort out what was available.

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Re: 40 amp fuse question; blade vs bolt down

Post by john61ct » Dec 08 2021 11:58am

Yes Peter is a really top notch vendor, congrats on going Quality.

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Re: 40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 09 2021 12:50pm

As I novice, I have no clear idea what kind of box to use (or how to mount the power resistor)? What is the best thing to put on the floor of a car that will stay cool and be safe. The components I have are:
1) a 4.8 " long ANL fuse block
2) a 3"x3" Switch
3) a 2 Ohm 100 Watt power resistor (about 3" long)
(See pictures and diagram below.)Image

Should I get a metal box? Can I use some sort of insulator (phenolic?) or something to mount the power resistor on? Or some sort of metal or insulating stand off? I am thinking it is not a good idea to mount the resistor directly to the side of the box, because it could, in principle, get warm. (Although it is actually just part of a precharge bypass circuit that passes current briefly to the inverter before I connect the inverter directly to the 14 volt source.) By the way, this switch can actually handle both the precharge and the main circuit since it takes two inputs and is "make-before-break".

Anyway, I would love to get advice on what kind of box to get and the best way to mount these three things as safely as possible,
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Re: 40 amp fuse question; blade vs bolt down

Post by amberwolf » Dec 11 2021 2:23am

zacksc wrote:
Dec 06 2021 12:33pm
If I just replace the 30 amp with a 40 amp blade fuse in the same in-line fuse holder, I was concerned that maybe the fuse holder would tend to heat up and that might be bad??
That type of fuse holder isn't known for always having good contacts to hold the fuse with; this tends to cause high resistance at the connections, whcih heats up the whole fuse holder. That could cause the fuse to blow from the heat generated in the contacts plus that in the fuse itself.

But....it could be blowing because the inverter system is actually drawing more power than you think it is, for long enough to blow the fuse.

I wondered if it might be more prudent to go with 40 amp bolt-down type fuse
Generally, those will be better, if you have the space for the holder. I use a bolt down fuse in the SB Cruiser's battery; in my case it's bolted inline with the wiring with no holder, and just thick heatshrink around it.

I know next to nothing about fuses and I have very little experience. So any advice about what is safest, most prudent, best, etc., is most welcome. Thanks very much!
A fuse is generally rated for the continuous current it should be able to sustain without blowing. Manufacturers have charts to go by for different uses and types, so I would measure what the actual continuous current is (not just calculated), and also what the peak current is, like during inverter startup, when the load turns on and off, etc. Then base the fuse rating you need to get on those.

The fuse is intended to protect the source (wiring, etc) against a failure in the load (inverter, etc) from causing a fire. So you want to size it such that it will blow before the wiring overheats (any wiring between battery and inverter), in the event of a failure in the inverter system.

The estimate of 25 amps at 13 volts is based on assuming 82% efficiency for the entire chain. Assuming 77% efficiency instead, the inferred current on the 13 volt side would be 26 amps. (13a x 26v = 338 Watts) I have note measured the current directly.
To be sure you are sizing the fuse correctly, I highly recommend measuring the actual current, as noted above.
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Re: 40 amp fuse question; blade vs bolt down

Post by amberwolf » Dec 11 2021 2:35am

zacksc wrote:
Dec 09 2021 12:50pm
As I novice, I have no clear idea what kind of box to use (or how to mount the power resistor)? What is the best thing to put on the floor of a car that will stay cool and be safe. The components I have are:
1) a 4.8 " long ANL fuse block
2) a 3"x3" Switch
3) a 2 Ohm 100 Watt power resistor (about 3" long)
(See pictures and diagram below.)
The resistor shoudln't be hot very long, as it wont' be in operation more than however long it takes to charge the caps in the inverter. If you want to find out how hot it will actually get, to know what you can mount it on safely, you can use an IR thermometer gun (or your fingers if you don't mind the slight risk of a burn).

If it doesn't get hot, it could be bolted to a plastic project box, along with the switch and fuse block.

If it does get hot, then a metal box might be a better option; they make die cast and aluminum project boxes of many sizes.

Presumably only the positive side wiring has to go thru any of these parts, so you won't have both battery positive and negative in the same place, but you do want to ensure this is all kept away from any metal frame of the vehicle chassis, as that is usually battery negative.

If you have to put all those things in the same box, I'd wire it so the fuse is the first thing after the battery wire enters the box. If you can put the fuse earlier in the wire, as close as possible to the battery, I'd do that as it then protects better (protecting against any short in any of the wiring between battery and inverter).
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Re: 40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 13 2021 12:12am

Thanks Amberwolf. Yes, the only the positive side goes through the switch box, and, as you said, current goes through the power resistor only briefly when the capacitors in the inverter are being charged. I am thinking I may try putting the power resistor on top of the ANL fuse box cover (as shown in the picture). Is that crazy?

I plan to use 8 AWG wire with 5/16" lugs (8 mm posts on the EV) and then fuse it at 40 amps. Does that seem okay? I could pay another $20 for a 35 amp fuse if that would be safer? Is that worth it? Advice is welcome.

I bought an aluminum box to put everything in. Shown here.
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Re: 40 amp fuse question; blade vs bolt down

Post by amberwolf » Dec 13 2021 12:52am

zacksc wrote:
Dec 13 2021 12:12am
I am thinking I may try putting the power resistor on top of the ANL fuse box cover (as shown in the picture). Is that crazy?
I would mount the resistor directly to the metal box, simply to ensure it never gets hot, and for a more sturdy mount.

While it's unlikely to be a problem, if it's mounted to the fuse box cover (which looks like plastic) then if that is soft enough plastic it may deform a little (more) each time the precharge is used, if enough heat is generated. How much heat will be generated I don't know (it can be calculated, I just don't remember the math); it is probably not much, but it has the potential to be.

Also, if the fuse cover is just held on by friction, not screwed down, it could come off and leave the precharge resistor floating around in the case.

I'd mount the switch on the outside of the box, if possible, so that the box can be closed in operation but still switched easily. If the shape of the "face" of the black part of the switch is flat, then you can just make a round hole big enough for the red switch handle in the box cover or side or end (whatever's easy for you), and then all the wiring and switch mechanics are contained in teh box.
I plan to use 8 AWG wire with 5/16" lugs (8 mm posts on the EV) and then fuse it at 40 amps. Does that seem okay? I could pay another $20 for a 35 amp fuse if that would be safer? Is that worth it? Advice is welcome.
I'd still stick the fuse at the battery (or whatever the source point is for the wiring to the inverter), rather than in the box. If the box is at the source point (rather than at the inverter), then that does the same job so the fuse can go in the box. If the box is in the middle of the wiring, or at the inverter end, it leaves however much wire is between the box and the source unprotected (against some types of failures). (the fuse isn't there (just) to protect the inverter, it's there to protect the wiring and system supplying the power).

The fuse rating depends on the current the system needs. If you don't know what the system is actually going to use (if it hasn't been measured), then you can only make educated guesses on the rating. What the ratings themselves mean you need to look at the manufacturer charts for the specific fuse you are going to use, to know when it is actually going to blow, and what it will allow to flow before that happens. If you specify exactly which brand and model fuse you have, we can find the chart for it. Some examples:
https://www.google.com/search?q=fuse+ra ... t&tbm=isch
Some more technical (but more specific) application notes from Littelfuse:
https://www.littelfuse.com/~/media/elec ... n_note.pdf
Example from that document:
1. NORMAL OPERATING CURRENT: The current rating of a fuse is typically derated 25% for operation at 25ºC to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25ºC ambient.

Do you have a wattmeter at all, for ebike stuff? If so, you can hook it up to this to measure the current draw. If not, you could temporarily leave the precharge resistor in series with the inverter power input, bolted to the aluminum box to help dissipate the heat, setup your voltmeter to 2VDC (or 20VDC if that's it's lowest range) and put it's red lead on one end of the resistor, black on the other, then measure the voltage as you turn on the inverter and then let it run at the max loading it will ever see, for a moment (just long enough for a stable reading).

Then turn it off, and change the voltmeter to 20Ohms, and note down the actual resistance reading you get while it's still warm (or hot).

Then you can use the formula of Volts / Resistance = Amps to figure out the current draw of the inverter.

Then you'll know for sure what the actual current draw is, and can size your fuse above that to never blow the fuse in use, but only in case of failure. :)
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Re: 40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 13 2021 7:08pm

Thanks a lot! I see what you mean. The distance between the 14 volt source and the fuse is a concern. Maybe I need to mount the ANL fuse box by itself right by the 14 volt source, and then have the switch and precharge resistor in a separate box a foot or two away?

I illustrated that here in this picture of the backseat area (with the rear bench taken out). The 14 volts comes from an 8 mm stud in the red "jacket" at the lower left in the picture.

Should I put the fuse right there (as illustrated)? Would I need a box for the fuse holder, or would the Blue Sea systems fuse holder with its insulating cover be okay on its own? (It is about 4.5 inches long.)
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Re: 40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 13 2021 7:12pm

Blue Sea Systems fuse Box
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Re: 40 amp fuse question; blade vs bolt down

Post by amberwolf » Dec 14 2021 12:35am

zacksc wrote:
Dec 13 2021 7:08pm
Thanks a lot! I see what you mean. The distance between the 14 volt source and the fuse is a concern. Maybe I need to mount the ANL fuse box by itself right by the 14 volt source, and then have the switch and precharge resistor in a separate box a foot or two away?

I illustrated that here in this picture of the backseat area (with the rear bench taken out). The 14 volts comes from an 8 mm stud in the red "jacket" at the lower left in the picture.

Should I put the fuse right there (as illustrated)?
It's safer for the wiring and such to have the fuse "at" the source, or as close as possible. Then if there is any short in the wiring to system ground, etc., the fuse blowing will protect all the wiring between the source and the short.

(if the fuse is farther away, then any short (say, from vibration wearing away insulation and allowing short to chassis ground, etc) in teh wiring between fuse and source isn't protected by that fuse. There is probably some form of protection within the system up to that stud, but it might not be user-accessible / resettable, and then you're stuck without 14v (possibly everwhere depending on protection system) until you can get it fixed. )

A problem is not likely to happen...but it's always possible. Ounce of prevention and all that. ;)

Would I need a box for the fuse holder, or would the Blue Sea systems fuse holder with its insulating cover be okay on its own? (It is about 4.5 inches long.)
If there's a place to tuck it in down in the "bay" there, it'll probably be fine just with it's own cover, as long as that completely shields all of the electrical connections and is secure (cover can't vibrate or fall off, etc).

If it can be bolted down that's even better.
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Re: 40 amp fuse question; blade vs bolt down

Post by zacksc » Dec 30 2021 5:10pm

Thank you. This is really helpful

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